The invention relates to a method for making sized paper or paperboard. In this kind of product, the goal is to improve the web strength by internal sizing of the web or by subjecting the web to surface sizing. Generally, a major portion of the size is starch, and sizing can substantially improve such qualities as the surface strength of the sized web, reduce its dusting propensity and increase its fiexural stiffness.
The invention also relates to an assembly suited for implementing the method.
In the treatment of fine paper grades sizing forms an important step in the manufacture, like in manufacture of the liner web of corrugated board and fluting, by substantially controlling the strength properties of the finished product. Generally, size has been applied to the web surfaces and, with the increasing interest to the manufacture of multilayer products, also to the middle layers of the product in the core thereof. In fine printing papers, the function of size is to improve the imprinting qualities of the paper web surface by virtue of giving the product a higher surface strength for better durability under the stresses of a printing process and reduced dusting propensity when used in a copier machine, for instance. Different kinds of starch are generally used as size, complemented with a variety of additives. However, since the present invention is not limited to any particular size composition, size must be understood in this context to refer to all compositions that are at least partially absorbable in the base web to be treated and serve to improve the strength of the base web.
Size is conventionally applied to the web as a dilute low-solids aqueous furnish. A major complication in the efficiency improvement of machines used for making fine papers and paperboard appears to be the drainage capacity of water transported into the web along with the size furnish. Furthermore, the drying of the formed web into an end product suited for making paper or paperboard requires a substantial portion in the overall energy budget of a paper mill. Inasmuch only a limited amount of water can be removed from a moving web by a single dryer, the number of successive dryer units must be increased in proportion to the elevated web speed. The larger number of drying equipment, such as dryer cylinders for instance, drastically increases the length of the papermaking machine and, in particular, its price, whereby the acquisition of a new high-speed line for making paperboard or fine paper grades may rise so high that an investment decisions becomes futile. On the other hand, the web speed of existing machinery is limited by the available drying capacity that curtails the maximum running speed and, hence, the potential production capacity.